Audit is formal & done by internal Quality assurance dept of organization or by external auditors but is for sure done by people/group not from project.------my project got audited last week and it was red in some areas and you can guess what might have happened after that

walk through is informal, no hard & fast set criteria or evaluation. i am preparing for another round of audit in next quarter and often using walk through as one technique to help me in assessing and moving red areas to green